Ah, I thought the explosions and the 'jinx' could be referencing CertainlyT's champion until the Pool Party group photo came out. The fact that she's chilling with Ziggs and the bullet tattoos on her arm show that she's highly likely to be related to explosions, while the air/wind puns are explained by the wind tattoos.

The "My next champion will be storming" is definitely about CertainlyT's champion (hence the "my"), and the "up in the air" pun seems to fit with "storm." It's hard to sort out what is what, and I don't know which champion is in the pool party splash, but that thread with CertainlyT, Xelnath and Gypsylord's comments was explicitly talking about two different champions, so we can't definitely assign all the puns to one champion. If I were to guess, I'd say we have an explosion-based champion and a lightning mage coming up.

The "My next champion will be storming" is definitely about CertainlyT's champion (hence the "my"), and the "up in the air" pun seems to fit with "storm." It's hard to sort out what is what, and I don't know which champion is in the pool party splash, but that thread with CertainlyT, Xelnath and Gypsylord's comments was explicitly talking about two different champions, so we can't definitely assign all the puns to one champion. If I were to guess, I'd say we have an explosion-based champion and a lightning mage coming up.

While it seems to fit CertainlyT's champ, the splash art clearly shows cloud tattoos on the person's arm, the bullets, and her chilling with Ziggs. It seems that she would be more likely to be air themed, but the idea of her using explosions isn't out of the picture either.
However, if that's not the case, then the next champion is bound to be storm or explosion themed, depending on what Vi's sister turns out to be.